This invention relates to a process for the manufacture of products from silicone rubber stock and, more particularly, from one-part silicone rubber stock. Products manufactured from silicone rubber stock are used, for example, in the automotive industry and the biomedical arts.
Industry currently employs primarily two types of silicone rubbers: heat-curing silicone rubbers and room-temperature vulcanizing (RTV) silicone rubbers. For the heat-curing silicone rubbers, the compounds are cured by heating with a free-radical generator, such as an organic peroxide, which acts as a cross-linking agent. These compounds are processed by molding (injection, transfer, or compression), extruding, calendering, dispersion, or sponging. The RTV silicone rubbers are cured either by condensation or by an addition reaction. In the condensation cure, either a one-part or two-part system can be used.
An example of a one-part silicone rubber stock is described in U.S. Pat. No. 3,070,559 to Nitzsche and Wick. This composition consists of organosiloxane components (i.e., organosiloxane monomers), fillers, cross-linking agents consisting of alkyoxy or aryloxysilanes or their condensation products, and Lewis acid or base catalysts. This patent teaches that either the cross-linking agent or catalyst, or both, can be isolated by absorption in an aluminum silicate molecular cage. It is necessary to provide a hydrophobic coating on the aluminum silicate after the cross-linking agent and catalyst have been absorbed. This acts to inhibit vulcanization brought about by water vapor in the air.
The two-part systems are composed of silicone rubber components (or silicone monomers) mixed with fillers and ethyl silicate. Just before using the material, the second part, a metal soap curing agent, is stirred in. This curing agent could, for example, consist of dibutyl tin dilaurate or tin octoate. The resultant reaction which causes the elimination of ethyl alcohol initiates curing. Modern Plastics Encyclopedia 1981-1982, Vol. 58, No. 10A, pp. 108-110.
The present invention is illustrated for use with heat-curing silicone rubber systems; however, it may also be applied to RTV silicone rubber systems.
As presently known, most heat-curing silicone rubber stock is made up intermittently in large batches according to production demand. After mixing, they are stored under controlled conditions until they are used. Because the cross-linking agent is mixed into the stock compound, the stock is partially cured and gelled during mixing, which increases its viscosity. It continues as well to cure slowly during storage. Thus, after one or two months in storage, it becomes too cured or viscous to use, and must be discarded. These stocks or compounds, therefore, have limited shelf life, depending upon storage conditions.
On the other hand, the stock may be mixed up on a smaller demand basis. While this solves the problem of viscosity and of product waste, it requires the presence of a formulating supervisor and the additional step of mixing before a production run.
In the course of mixing the silicone rubber systems, the temperature of the stock increases because of kinetic energy transferred to the stock and because the reaction is exothermic. The intensity and length of the mixing process must be restricted to avoid excessive premature cure, since the additional heat causes the reaction between the available cross-linking agent and the silicone components. Water jacket cooling techniques may be employed to inhibit this reaction, but the mixing operation as currently practiced remains an art employing variations of elastomeric components and cross-linking agents, as well as use of promoters, accelerators, and inhibitors. The accelerators used may be, for example, heavy metal salts, metal soaps such as cobalt naphthenate, manganese octenate, tertiary amines such as dimethylanilines, mercaptanes, and azo compounds.
Often, the mixing process with these known stocks must be terminated without regard for the uniformity of distribution. Thus, the problem occurs that because of the restricted conditions under which the stocks are mixed, the cross-linking agent, elastomers, and other additives are not completely distributed throughout the mixture. This frequently occurs if the temperature of the mixture increases too much and the mixing operation has to be cut short before the additives are completely mixed in.
For products molded from silicone rubber stock, there have always been problems in filling the molds completely and obtaining suitable surface finish of the molded parts. Particularly for the medical field, products frequently have to be hand-finished, which is both expensive and time-consuming.
Organic resin compositions containing encapsulated catalysts are known. U.S. Pat. No. 4,362,566 to Hinterwalder, for example, teaches the use of encapsulation to inhibit the reaction in a polymeric system. For the most part, such capsules are intended to shatter with the application of mechanical forces, and in particular mechanical shearing forces. This would prove a significant problem during the mixing stage of the present system.
The practices for microencapsulation of materials are well known, and have been well known since the end of World War II. Reference is made to "Microcapsule Processing and Technology," Asagi Kondo, edited and revised by J. Wade Van Valkenberg, Marcel Decker, Inc., New York, N.Y. (1979), and "Capsule Technology in Microencapsulation," Noise Data, Parkridge, N.J. (1972). The cross-linking agents used are usually organic peroxides in the form of volatile liquids or solids dissolved in a solvent. These cross-linking agents are both toxic and difficult to handle.